Convert to image_utils for qemu-img info and resize

Code added for GlusterFS snapshot support in Havana included
new code to parse qemu-img output.  This removes this code
and uses image_utils qemu_img_info instead.

Improve GlusterFS driver test coverage with tests for cloning,
create from snapshot, and initialize_connection.

Add test to image_utils for parsing a different style of
qemu-img info output.

Change-Id: I8f1811b400c06edb3cd7416c52aa297921841e54
GlusterFS: Use correct source file in copy_volume_from_snapshot

The GlusterFS driver's copy_volume_from_snapshot was previously
not parsing the snapshot information to determine which backing
file to copy from, instead copying from the active file at all
times.  This would result in the wrong data being supplied when
a volume is cloned from a snapshot.

Closes-Bug: 1222907

Change-Id: Ib829ca1a6812b61845f3b2eb9c5507779fa6ec15